It is well-known that vehicles having spaced apart drive wheels or wheel sets powered by a single engine through a differential mechanism are particularly problematic when one of the differentially driven wheels or wheel sets loses traction. Conditions which give rise to a loss of traction are commonly found in construction sites, mines, and other off-road situations. A vehicle having one of two differentially driven wheels or wheel sets located on a slippery surface and the other located on a surface providing good traction is often unable to move, owing to the fact that the differential mechanism directs full engine power to the wheel having no appreciable traction. The result is a slip condition in which the wheel having no traction rotates at higher than normal speed and the wheel having traction remains stationary.
To alleviate such problems, various mechanical and electro-mechanical anti-spin devices have been developed and placed in commercial service. A particularly advantageous anti-spin system utilizes electronics to supply a braking force to the slipping or spinning wheel. The application of braking force to the slipping wheel simulates increased traction at that wheel and results in a more even distribution of power between the differentially driven wheels. An effective example of this approach is described in U.S. Pat. No. 4,344,149, issued to Miller et al. on Aug. 10, 1982, and assigned to the assignee of the instant application. Miller discloses an apparatus for applying a proportionally variable braking force to the wheel which loses traction, during a slip control time period. A slip signal is produced responsive to any difference between the rotational velocity of the differentially driven wheels, and the slip signal is compared with a predetermined reference signal. In response to the slip signal exceeding the reference signal, the system selectively applies a braking force to the faster turning wheel. The braking force is modulated proportionally according to the degree of slip represented by the slip signal.
The Miller electronic approach to anti-spin control offers numerous technical advantages over earlier pure mechanical systems. However, like various other known electronic control systems, under defined conditions the Miller system suffers from one particular problem. Most off-road vehicles include spring applied parking brakes that are maintained in a released position by the application of hydraulic oil pressure. In order to engage such brakes, a substantial amount of oil must be exhausted from the system before the brake shoes make contact with the friction surface. Therefore, a significant amount of time elapses between the command to apply the brakes and the actual application of braking force. The inherent delay in applying the parking brakes is normally of no consequence. However, when these brakes are utilized to exercise dynamic control over a spinning wheel, the pressure induced delays can significantly affect the effectiveness of the system.
In a typical operating condition in which slippery surfaces are encountered, traction is often less than dependable with respect to both driven wheels of the vehicle. Frequently, the particular wheel having the best traction varies from time-to-time while traversing the terrain. Under such conditions, it is desirable to pre-excite both brake mechanisms to a point just short of actual brake engagement. Such pre-excitation exhausts the large volume of excess oil maintaining the opposing brake sets in the fully released position, and prepares the brakes for rapid and responsive application as required. By pre-exciting the brakes, the control system is able to provide continuous and well modulated control over both driven wheels, with minimal time delays caused by the need to exhaust oil from the system and with virtually no time delay in transferring braking action from one vehicle wheel to the other as traction conditions fluctuate.
The present invention is directed to overcoming one or more of the problems as set forth above.